286 COMPARATIVE ANATOMY 



Accessory Organs in Connection with the Eye. 



(a) EYE-MUSCLES. 



The movement of the eye-ball is in general (except in Myxinoids, 

 cf. p. 278) effected by six muscles, four of which are known as the 

 recti (superior, inferior, external or posterior, and internal or anterior), 

 and two as the obliqui (superior and inferior). The former, which 

 arise from the inner portion of the orbit, usually from the dural 

 sheath of the optic nerve, together circumscribe a pyramidal cavity, 

 the apex of which lies against the inner portion of the orbit, while 

 the base surrounds the equator of the eye-ball, where the muscles 

 are inserted into the sclerotic. 



Both the oblique muscles usually arise from the anterior or nasal 

 side of the orbit, and as they respectively pass from this region 

 dorsally and ventrally in an equatorial direction round the eye-ball, 

 they constitute a sort of incomplete muscular ring. 1 



Besides these six muscles, others are usually present from the 

 Amphibia onwards. Of these, the retractor lulbi, which often consists 

 of several portions, is derived genetically from the posterior rectus, 

 and is supplied by the abducent nerve. In Amphibians, Reptiles, 

 and Birds, a portion of the striated masticatory musculature, 

 supplied by the trigeminal, extends into the fibrous walls of the per- 

 iorbita(p. 276), and in Rana, for example, gives rise to an elevator of 

 the eye-ball, a depressor of the lower eyelid, and a kind of extensor 

 of the lower wall of the orbital sac ; the connection of which with 

 the masticatory muscles is plainly seen. In Anurans there is also a 

 more ventral, transverse layer, corresponding to the depressor of the 

 lower lid in Lizards and Birds, in which latter there is also a layer 

 in the orbital membrane corresponding to the depressor of the lower 

 orbital wall in Amphibians. 



As the transversely striped periorbital muscles gradually be- 

 come of less importance in the Vertebrate series, the smooth 

 muscles are further developed. The latter are already indicated in 

 Teleosts ; but in Reptiles (especially Lizards and Chelonians) are 

 much more marked, and are continued into the eyelids. In 

 Mammals, the development of the smooth orbital musculature varies 

 considerably, and is less marked the more the orbit is enclosed by 

 bone (e.g. in Primates, in which it lies in the infra-orbital fissure). 

 It, like the upper and lower palpebral muscles, which are 

 specially important in Mammals, are supplied by the sympathetic. 



1 A deviation from this arrangement is seen in Mammals, in which the 

 superior oblique has gradually come to arise from the inner part of the orbit, and 

 then passes forwards towards its anterior (internal) angle, where it becomes 

 tendinous, and passes through a fibro- cartilaginous pulley (trochlea) attached to 

 the upper border of the orbit, on the frontal bone. Hence it is sometimes called 

 the trochlear muscle. From this point it changes its direction, and becomes 

 reflected obliquely outwards and backwards to the globe of the eye. 



